In conventional semiconductor memories, one bit of information is represented by electrons stored in a static capacitor in each memory cell. The binary number "1" is represented by a deficit of N electrons and "0" is represented by a neutral charge state. In a typical 16 Mbit dynamic random access memory (DRAM), the number of electrons N is around 800,000. In order to increase the memory capacity, the individual memory cells need to be made smaller, but this cannot be achieved simply by scaling down the conventional memory cell because there is a lower limit to the value of N. The number of electrons N is limited by the need to accommodate leakage current from the cell, internal noise and the effect of incident alpha particles, and these factors do not reduce commensurately with a reduction in the area of the memory cell. It can be estimated that N must be in excess of 130,000 in a 16 Gbit DRAM i.e a factor of approximately 6 times less than for a 16 Mbit DRAM. However, the cell size required for a 16 Gbit DRAM needs to reduce by the factor of three orders of magnitude as compared with a 16 Mbit DRAM and consequently, the reduced cell size cannot accommodate the number of electrons required for satisfactory operation. In an attempt to maintain the value of N sufficiently large, three dimensional capacitors with trench or stacked structures, together with high dielectric capacitor films have been investigated but the resulting proposed structures and fabrication processes become extremely complicated. Furthermore, the power consumption increases significantly because the relatively large number N of electrons in the cells need to be refreshed within a storage time which tends to become shorter as the scale of the device is miniaturised.
Another type of memory device is known as a flash memory, which exhibits non-volatile characteristics. In such a device, approximately 10.sup.5 electrons are injected into a floating gate through a tunnelling barrier, typically formed of SiO.sub.2 with a thickness of the order of 10 nm. The stored charge produces a field which influences current flow in a source-drain path. Charge is either written to or erased from the floating gate by application of an electric field through a control gate. A relatively high electric field is applied during the erase and write cycles and as a result the SiO.sub.2 film is degraded, limiting the life of the memory to a predetermined number of erase/write cycles, typically of the order of 10.sup.5 cycles. Furthermore, the erase/write times are typically several milliseconds, four orders of magnitude slower than that of a conventional DRAM. Such poor performance limits the application of flash memory devices.
Hitherto, alternative approaches have been proposed to provide memory devices which operate with small, precise numbers of electrons, known as single electron memory devices. A single electron memory device is described in our PCT/GB93/02581 (WO-A-94/15340). A precise number of electrons enter or leave a memory node through a multiple tunnel junction under the control of applied gate voltages and the electron state at the memory node is detected by means of an electrometer. However, a disadvantage of the device is that a significant amount of circuitry is required for each memory node and the device currently operates only at low temperature, below the liquid helium temperature of 4.2K. Another single electron memory device has been proposed and demonstrated by K. Yano, T. Ishii, T. Hashimoto, T. Kobayasi, F. Murai and K. Seki in IEEE Transactions on Electron Devices, September 1994, Vol. 41, No. 9, pp. 1628-1638, and by K. Yano, T. Ishii, T. Sano, T. Mine, F. Murai and K. Seki in 1996 IEEE International Solid-State Circuits Conference, 1996, FP 16.4, p. 266. The device utilises a polycrystalline film extending between a source and drain, to which a gate voltage is applied. A small number of electrons is stored in the granular structure of the polycrystalline silicon film. The memory size is relatively small as compared with the structure in PCT/GB93/02581 supra and is operable at room temperature. Furthermore, the memory shows several advantages as compared with conventional flash memory, with a faster erase/write time due to the small number of stored electrons, and the operational lifetime is improved because low-voltage tunnel injection is utilised rather than high-field electron injection. However, the time to read stored information is relatively long, of the order of several microseconds, because the resistance between the source and drain needs to be sufficiently high to ensure long storage time of electrons in the grains.
Another structure is described by S. Tiwari, F. Rana, H. Hanafi, A. Hartstein, E. F. Crabbe and K. Chan in Applied Physics Letters, Mar. 4, 1996, Vol 68, No. 10, pp. 1377-1379, by S. Tiwari, F. Rana, K. Chan, L. Shi and H. Hanafi in Applied Physics Letters, Aug. 26, 1996, Vol 69, No. 9, pp. 1232-1234, and H. I. Hanafi, S. Tiwari and I. Khan in IEEE Transactions on Electron Devices, Sep. 9, 1996, Vol 43, No. 9, pp 1553-1558. This memory device utilises silicon-crystals that underlie the gate of a transistor device. Electrons are injected into the silicon nano-crystals, which are 5 nm in dimension, from the silicon substrate through a thin tunnelling oxide layer of the order of 1.1-1.8 nm thickness. Stored electrons shift the threshold voltage of the transistor. The time to read stored information is relatively short, of the order of several tens of nanoseconds, because the transistor channel has a high electron mobility. The endurance cycle for writing and erasing information is significantly improved relative to a conventional flash memory device. However, the erase time is unsatisfactorily long, of the order of several milliseconds because the conduction band alignment is unfavourable for electrons to tunnel from the nano-crystals into the bulk silicon.
Another memory device which operates according to the principles of flash memory is disclosed in Electrically-Alterable Memory Using a Dual Electron Injector Structure, D. J. DiMaria, K. M. DeMeyer and D. W. Dong, IEEE Electron Device Letters, Vol. EDL-1, No.9, September 1980, pp.179-181. In this device, the conductivity of the source/drain path is controlled by charge written or erased from a floating gate through a tunnelling barrier from a gate electrode. However, disadvantages of this device are that it has a slow writing/erasing time, of the order of milliseconds, and that the life of the tunnel barrier is limited because Fowler-Nordheim high field injection is used as in a conventional flash memory. A similar device is described in U.S. Pat. No. 3,878,549 to S. Yamazaki.